We are examining in physiological terms the postnatal development of cognitive functions in the dorsolateral prefrontal association cortex of rhesus monkeys. Immature and adult monkeys are trained to perform a delayed response task which is known to be an indicator of prefrontal function in the adult animal. Utilizing local, reversible cooling adapted to chronic, unanesthetized animals, we are able to examine the effects of temporary cryogenic depression of neural function in the dorsolateral cortex at various times during each animal's period of maturation. This has led to the demonstration of a gradual increase (over the first 3 years of life) in dependence upon this cortical field for cognitive functions critical to delayed response performance. Single neuron recordings which we have obtained from this region during such performance are consistent with such a maturation process. Recordings of this kind are also being made from two subcortical structures with known connections with the dorsolateral prefrontal cortex. Reversible cryogenic depression of the dorsolateral cortex during the subcortical recordings is permitting us to examine the emergence during development of prefrontal influences carried over efferent pathways.